Method for manufacturing a semiconductor device having a super junction MOSFET

ABSTRACT

A method of manufacturing a super junction MOSFET, which includes a parallel pn layer including a plurality of pn junctions and in which an n-type drift region and a p-type partition region interposed between the pn junctions are alternately arranged and contact each other, a MOS gate structure on the surface of the parallel pn layer, and an n-type buffer layer in contact with an opposite main surface. The impurity concentration of the buffer layer is equal to or less than that of the n-type drift region. At least one of the p-type partition regions in the parallel pn layer is replaced with an n −  region with a lower impurity concentration than the n-type drift region.

This is a division of U.S. patent application Ser. No. 14/882,423, filedon Oct. 13, 2015, and allowed on Mar. 29, 2017, which is a continuationunder 35 U.S.C. 120 of International Application PCT/JP2014/068632having the International Filing Date of Jul. 11, 2014, and having thebenefit of the earlier filing date of Japanese Application No.2013-192789, filed on Sep. 18, 2013. Each of the identified applicationsis fully incorporated herein by reference.

BACKGROUND Technical Field

The present invention relates to a semiconductor device (insulated gatefield effect transistor) and a method for manufacturing the same.

Background Art

For example, a metal-oxide-semiconductor field effect transistor(MOSFET: insulated gate field effect transistor) or an insulated gatebipolar transistor (IGBT) has been known as a semiconductor element usedin a power semiconductor device. FIG. 5 is a circuit diagramillustrating a general inverter. FIG. 6A is a cross-sectional viewillustrating a main portion of a general IGBT and FIG. 6B is across-sectional view illustrating a main portion of a MOSFET. An IGBT101 has come into widespread use as a high-breakdown-voltage switchingelement which is used for an inverter circuit 1000 illustrated in FIG.5. The IGBT 101 has excellent features, such as the high breakdownvoltage and low on-voltage of a bipolar transistor, or excellentfeatures, such as a lower speed than the MOSFET and a high-speedoperation, and is an important semiconductor element which supportspower electronics now.

However, the IGBT 101 illustrated in the main portion cross-sectionalview of FIG. 6A has a reverse breakdown voltage junction (collectorjunction 103), unlike a MOSFET 301 illustrated in FIG. 6B. Therefore, ingeneral, in the IGBT 101, a current cannot flow in a reverse direction(a bias direction in which an emitter E is a positive electrode and acollector C is a negative electrode). When the IGBT 101 is changed froman on state to a forward blocking state, a high surge voltage is likelyto be generated in the reverse direction due to an inductance componentin the circuit. When the surge voltage is applied to the IGBT 101,generally, there is a concern that the IGBT 101 which is not protectedfrom a reverse breakdown voltage will be broken. However, when the IGBTis used in the inverter circuit, the IGBT is protected by a diode 401(see FIG. 5) which is connected in inversely parallel in order to returnan L load (dielectric load) current that is generated whenever the IGBT101 is turned off. Reference numerals 102 and 302 indicate an n⁻ driftlayer.

There is an increasing demand for increasing the frequency of theinverter. The parallel connection of the IGBT 101 and the generalfree-wheeling diode 401 has a limitation in increasing a switchingspeed. Therefore, the IGBT 101 which can switch at a high speed and afast diode are used in order to meet the demand. In the fast diode, thetime required for reverse recovery when the diode is changed from astate in which a forward current flows to a reverse blocking state isshorter than that of a general diode. The use of the diode makes itpossible to reduce reverse recovery loss.

FIG. 2A is a cross-sectional view illustrating a main portion of a superjunction MOSFET according to the related art and is a carrier lifetimedistribution diagram in which the vertical axis indicates a depth incorrespondence with the depth direction of a substrate corresponding toFIG. 2A. In recent years, in order to further improve the speed of theswitching element, the replacement of the IGBT 101 with a super junctionMOSFET 201 illustrated in FIG. 2A has been examined. The super junctionMOSFET 201 (see FIG. 2A) which has been examined as a replacement targethas a super junction (SJ) structure having a drift layer 205 as aparallel pn layer in which a n-type region (hereinafter, referred to asan n-type drift region) 202 a with high impurity concentration and ap-type region (hereinafter, referred to as a p-type partition region)202 b are alternately arranged in a direction parallel to a main surfaceof a substrate at a small interval (pitch). In addition, the drift layerincludes a first n-type buffer layer 204 which is provided on the drainside of a parallel pn layer 202. When the carrier lifetime of thesubstrate is not controlled, the carrier lifetime is constant (notcontrolled) in the depth direction from the surface of the substrate, asillustrated in FIG. 2B. In the super junction MOSFET 201, even whenimpurity concentration is higher than general impurity concentration inorder to match the n-type drift region 202 a of the parallel pn layer202 with the breakdown voltage, the pitch between the parallel pn layers202 can be reduced to deplete all of the parallel pn layers 202 at a lowvoltage. Therefore, the super junction MOSFET 201 has thecharacteristics of a high-breakdown voltage and low on-resistance eventhough it is a unipolar type. In addition, the super junction MOSFET canperform high-speed switching resulting from a unipolar device andincludes a reverse diode structure (reference numerals 203 and 202 a inFIG. 2(a)). Therefore, it is not necessary to newly connect the paralleldiode 401 of the inverter circuit illustrated in FIG. 5 and a reductionin the size of the device can be expected. In addition, the superjunction MOSFET (SJ-MOSFET) 201 is used as a switching device and thebuilt-in diode is used as a fast recovery diode to further increase thespeed and to further reduce loss.

As a document related to the super junction MOSFET 201, a document whichdiscloses the following structure has been published: an SJ structureincluding a parallel pn layer and an n-type buffer layer which isprovided below the layer and in which impurity concentration is changedin two stages are provided in a drift layer 205 to reduce on-resistanceand to form a built-in diode having a soft recovery waveform as reverserecovery characteristics (for example, see the following Patent Document1). In addition, a semiconductor device has been known which has anSJ-MOS structure for shortening a reverse recovery time, withoutincreasing a leakage current between a drain and a source (for example,see the following Patent Document 2). Furthermore, a structure has beenproposed in which an SJ-MOSFET is connected to a Schottky barrier diodehaving an SJ structure to achieve a semiconductor device suitable for asoft switching type (for example, see the following Patent Document 3).A structure has been proposed in which a lifetime control region isprovided in the entire Schottky barrier diode having an SJ structure toreduce a reverse current and to improve reverse recovery characteristics(for example, see the following Patent Document 4). A lifetime controlmethod for obtaining reverse recovery characteristics with a softrecovery waveform (for example, see the following Patent Document 5). Amethod for controlling the lifetime of an excess minority carrier hasbeen proposed (for example, see the following Patent Document 6). Inaddition, a semiconductor device has been proposed which can improve abreakdown voltage and turn-off characteristics, as compared to theelements according to the related art (for example, see the followingPatent Document 7).

CITATION LIST Patent Document

Patent Document 1: JP 2003-101022 A (FIG. 11 and Paragraphs [0077] to[1079])

Patent Document 2: Re-publication of PCT International Publication No.2010-24433 (Abstract)

Patent Document 3: JP 2006-24690 A (Object and Solving Means inAbstract)

Patent Document 4: JP 2008-258313 A (Abstract)

Patent Document 5: JP 2007-59801 A (Abstract)

Patent Document 6: JP 7-226405 A (Object)

Patent Document 7: JP 2001-102577 A (Object)

In the super junction MOSFET 201 illustrated in FIG. 2A, in the reverseblocking state, the depletion layer is spread into each column (then-type drift region 202 a and the p-type partition region 202 b) in theparallel pn layer at a low breakdown voltage and is completely depleted.At that time, the built-in diode (reference numerals 203-202 a) ischanged from the state in which a forward current (reflux current) tothe reverse bias blocking state (that is, a reverse recovery state) ofthe pn junction of the built-in diode. However, in the built-in diode inthe reverse recovery state, since the super junction MOSFET 201 has aunipolar structure, there are few minority carriers and a reverserecovery current Irp is small. Therefore, a so-called hard recoverywaveform in which a current waveform and a voltage waveform rise rapidlyis likely to be obtained. FIG. 3 which will be described below alsoillustrates the reverse recovery current waveform of the super junctionMOSFET having the structure according to the related art illustrated inFIGS. 2A and 2B. When a reverse recovery operation has a hard recoverywaveform, ringing (oscillating waveform) occurs, which causes noise, asillustrated in the reverse recovery waveform diagram of the superjunction MOSFET having the structure according to the related artillustrated in FIG. 3 (In FIG. 3, oscillatory waveform portions overlapeach other, look like a thick black line, and are indistinct). Thewaveform of the structure according to the related art illustrated inFIG. 3 is the simulation result of the current waveform of the reverserecovery operation when a power supply voltage is 400 V, a forwardcurrent is 20 A, and a change in reverse current over time is 100 A/μs,for the vertical super junction MOSFET 201 having the structureaccording to the related art illustrated in FIG. 2A.

SUMMARY

The invention has been made in view of the above-mentioned problems andan object of the invention is to provide a semiconductor device whichprevents a sharp rise in hard recovery waveform during a reverserecovery operation and a method for manufacturing the same. In addition,an object of the invention is to provide a semiconductor device whichcan prevent a sharp rise in hard recovery waveform to reduce a reverserecovery current (Irp) and a reverse recovery time (trr) and can achievehigh-speed switching and low reverse recovery loss and a method formanufacturing the same.

In order to solve the above-mentioned problems and achieve the object, asemiconductor device according to an aspect of the invention has thefollowing characteristics. A parallel pn layer is provided over a firstmain surface of a drain layer of a first conductivity type. The parallelpn layer includes a plurality of pn junctions which extend in a verticaldirection and are parallel to each other. A drift region of the firstconductivity type and a partition region of a second conductivity typewhich are interposed between the pn junctions are alternately arrangedso as to come into contact with each other. A MOS gate structure isprovided on a first main surface side of the parallel pn layer. A firstbuffer layer of the first conductivity type is provided between theparallel pn layer and the drain layer. The first buffer layer has alower impurity concentration than the drift region. At least one of thepartition regions in the parallel pn layer is replaced with afirst-conductivity-type region which has a lower impurity concentrationthan the drift region. A second buffer layer of the first conductivitytype may be provided between the first buffer layer and the drain layer.The second buffer layer may have a higher impurity concentration thanthe drift region. The parallel pn layer may have a lattice-shaped planepattern.

In order to solve the above-mentioned problems and achieve the object,according to another aspect of the invention, there is provided a methodfor manufacturing a semiconductor device including a high-concentrationbuffer layer of a first conductivity type which is provided on a firstmain surface of a drain layer of the first conductivity type and has ahigher impurity concentration than a drift region, a low-concentrationbuffer layer of the first conductivity type which is provided on thehigh-concentration buffer layer and has a lower impurity concentrationthan the drift region, and a parallel pn layer which is provided on thelow-concentration buffer layer and in which the drift region of thefirst conductivity type and a partition region of a second conductivitytype are alternately arranged, at least one of the partition regionsbeing replaced with a first-conductivity-type region having a lowerimpurity concentration than the drift region. The method includes a stepof adjusting a carrier lifetime of the parallel pn layer to be shorterthan the carrier lifetime of the high-concentration buffer layer, usingaddition of heavy metal or irradiation with a charged particle.

In order to solve the above-mentioned problems and achieve the object,according to still another aspect of the invention, there is provided amethod for manufacturing a semiconductor device including ahigh-concentration buffer layer of a first conductivity type which isprovided on a first main surface of a drain layer of the firstconductivity type and has a higher impurity concentration than a driftregion, a low-concentration buffer layer of the first conductivity typewhich is provided on the high-concentration buffer layer and has a lowerimpurity concentration than the drift region, and a parallel pn layerwhich is provided on the low-concentration buffer layer and in which thedrift region of the first conductivity type and a partition region of asecond conductivity type are alternately arranged, at least one of thepartition regions being replaced with a first-conductivity-type regionhaving a lower impurity concentration than the drift region. The methodhas the following characteristics. First, a step of forming thehigh-concentration buffer layer of the first conductivity type, whichhas a higher impurity concentration than the drift region, on the firstmain surface of the drain layer is performed. Then, a step of formingthe low-concentration buffer layer of the first conductivity type, whichhas a lower impurity concentration than the drift region, on thehigh-concentration buffer layer is performed. Then, a step of formingthe parallel pn layer on the low-concentration buffer layer isperformed. Then, a step of adding heavy metal or radiating a chargedparticle to the parallel pn layer such that a carrier lifetime of theparallel pn layer is shorter than the carrier lifetime of thehigh-concentration buffer layer is performed.

In order to solve the above-mentioned problems and achieve the object,according to yet another aspect of the invention, there is provided amethod for manufacturing a semiconductor device including ahigh-concentration buffer layer of a first conductivity type which isprovided on a first main surface of a drain layer of the firstconductivity type and has a higher impurity concentration than a driftregion, a low-concentration buffer layer of the first conductivity typewhich is provided on the high-concentration buffer layer and has a lowerimpurity concentration than the drift region, and a parallel pn layerwhich is provided on the low-concentration buffer layer and in which thedrift region of the first conductivity type and a partition region of asecond conductivity type are alternately arranged, at least one of thepartition regions being replaced with a first-conductivity-type regionhaving a lower impurity concentration than the drift region. The methodhas the following characteristics. First, a step of forming the parallelpn layer on a front surface side of a semiconductor substrate isperformed. Then, a step of forming an element structure on the parallelpn layer on the front surface side of the semiconductor substrate isperformed. Then, a step of forming the low-concentration buffer layer ofthe first conductivity type, which has a lower impurity concentrationthan the drift region, on a rear surface side of the semiconductorsubstrate is performed. Then, a step of forming the high-concentrationbuffer layer of the first conductivity type, which has a higher impurityconcentration than the drift region, at a position that is shallowerthan the low-concentration buffer layer from a rear surface of thesemiconductor substrate is performed. Then, a step of adding heavy metalor radiating a charged particle to the high-concentration buffer layersuch that a carrier lifetime of the parallel pn layer is shorter thanthe carrier lifetime of the high-concentration buffer layer isperformed.

According to the invention, it is possible to provide a semiconductordevice which prevents a sharp rise in hard recovery waveform during areverse recovery operation and a method for manufacturing the same. Inaddition, it is possible to provide a semiconductor device which canoperate at a high speed and reduce reverse recovery loss and a methodfor manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views illustrating a main portion ofa super junction MOSFET according to Embodiment 1 of the invention inwhich a p-type partition region in a parallel pn layer is replaced withan n-type region having a lower impurity concentration than an n-typedrift region;

FIGS. 2A and 2B are cross-sectional views illustrating a main portion ofa super junction MOSFET according to the related art and is a carrierlifetime distribution diagram in which the vertical axis indicates adepth in correspondence with a depth direction of a substratecorresponding to FIG. 2A;

FIG. 3 is a diagram illustrating reverse recovery current waveformscorresponding to the super junction MOSFET having the structureaccording to the related art illustrated in FIGS. 2A and 2B and thesuper junction MOSFET according to Embodiment 1 of the inventionillustrated in FIG. 1;

FIG. 4 is a diagram illustrating different carrier lifetimedistributions of the super junction MOSFET according to Embodiment 1 ofthe invention;

FIG. 5 is a circuit diagram illustrating a general inverter;

FIG. 6A is a cross-sectional view illustrating a main portion of ageneral IGBT and FIG. 6B is a cross-sectional view illustrating a mainportion of a MOSFET;

FIG. 7 is an example of a cross-sectional view illustrating a mainportion of the plane pattern of the parallel pn layer which is cut alongthe plane parallel to the surface of a substrate in the super junctionMOSFET according to the invention;

FIG. 8A is a cross-sectional view illustrating the main portion takenalong the dashed line B-B′ of FIG. 7 and FIG. 8B is a cross-sectionalview illustrating the main portion taken along the dashed line C-C′ ofFIG. 7;

FIG. 9 is another example of the cross-sectional view illustrating themain portion of the plane pattern of the parallel pn layer which is cutalong the plane parallel to the surface of the substrate in the superjunction MOSFET according to the invention; and

FIG. 10A is a cross-sectional view illustrating the main portion takenalong the dashed line B-B′ of FIG. 9 and FIG. 10B is a cross-sectionalview illustrating the main portion taken along the dashed line C-C′ ofFIG. 9.

DETAILED DESCRIPTION

Hereinafter, embodiments of a semiconductor device, a method formanufacturing the same, and a composite semiconductor device in which adiode is connected in parallel according to the invention will bedescribed in detail with reference to the accompanying drawings. In thespecification and the accompanying drawings, in the layers or regionshaving “n” or “p” appended thereto, an electron or a hole means amajority carrier. In addition, symbols “+” and “−” added to n or p meanthat impurity concentration is higher and lower than that of the layerwithout the symbols. In the description of the following embodiments andthe accompanying drawings, the same components are denoted by the samereference numerals and the description thereof will not be repeated. Inaddition, in the accompanying drawings described in the embodiments, forease of viewing or understanding, a scale and a dimensional ratio aredifferent from the actual scale and dimensional ratio. The invention isnot limited to the following embodiments as long as it does not departfrom the scope and spirit thereof.

Embodiment 1

FIGS. 1A and 1B are cross-sectional views illustrating a main part of asuper junction MOSFET according to Embodiment 1 of the invention inwhich a p-type partition region in a parallel pn layer is replaced withan n-type region having a lower impurity concentration than an n-typedrift region. FIGS. 1A and 1B are cross-sectional views illustrating amain part of an active portion of an element in each of vertical superjunction MOSFETs 50 and 51 according to the invention. The verticalsuper junction MOSFETs 50 and 51 illustrated in FIGS. 1A and 1B have asuper junction (SJ) structure in which a drift layer is a parallel pnlayer 4 including an n-type region (n-type drift region) 4 a with highimpurity concentration and a p-type region (p-type partition region) 4 bwhich are alternately arranged in a direction parallel to the mainsurface of a substrate. That is, the vertical super junction MOSFETs 50and 51 have a plurality of pn junctions 6 which are formed by the n-typedrift regions 4 a and the p-type partition regions 4 b forming theparallel pn layer 4, extend in a direction (the depth direction of thesubstrate) perpendicular to the main surface of the substrate, and areparallel to each other. FIG. 1A illustrates the vertical super junctionMOSFET 50 with an SJ structure in which a plurality of regions of somep-type partition regions 4 b in the parallel pn layer 4 are n⁻ regions 4c having a lower impurity concentration than the n-type drift region 4a. The vertical super junction MOSFET 50 includes a first n⁻ bufferlayer 3 having the same impurity concentration as the n⁻ region 4 c anda second n⁺ buffer layer 2 having a higher impurity concentration thanthe n-type drift region 4 a of the parallel pn layer 4 which arearranged in this order from the parallel pn layer 4 between the parallelpn layer 4 and the n⁺⁺ drain layer 1.

FIG. 1B illustrates the vertical super junction MOSFET 51 with an SJstructure in which one p-type partition region 4 b of the parallel pnlayer 4 is the n⁻ region 4 c having a lower impurity concentration thanthe n-type drift region 4 a. The vertical super junction MOSFET 51includes the first n⁻ buffer layer 3 which is provided between the lowersurface of the parallel pn layer 4 and the n⁺⁺ drain layer 1 and has thesame impurity concentration as the n⁻ region 4 c. In addition, thevertical super junction MOSFETs 50 and 51 each include a general MOSgate (metal-oxide-semiconductor insulated gate) structure including ap-type base region 5, an n⁺ source region 7, a p⁺ contact region 8, agate insulating film 9, and a gate electrode 11 and a source electrode12, which are provided on the side of the parallel pn layer 4 oppositeto the first n⁻ buffer layer 3. A drain electrode 13 comes into contactwith the n⁺⁺ drain layer 1. When the semiconductor device is turned on,a current flows in an active region (the active portion of the element)(the active region is in charge of current driving).

In each of the vertical super junction MOSFETs 50 and 51 illustrated inFIGS. 1A and 1B, some of the p-type partition regions 4 b in theparallel pn layer 4 are the n⁻ regions 4 c. Therefore, the verticalsuper junction MOSFETs 50 and 51 include pin diodes 10 a and 10 b andMOSFET regions 20, respectively. The pin diode 10 a includes the p-typebase region 5, the n⁻ region 4 c, the first n⁻ buffer layer 3, and thesecond n⁺ buffer layer 2 and the pin diode 10 b includes the p-type baseregion 5, the n⁻ region 4 c, and the first n⁻ buffer layer 3.

Since the vertical super junction MOSFETs 50 and 51 according to theinvention have the above-mentioned structures, it is possible to operatethe pin diodes 10 a and 10 b, without reducing a breakdown voltage. Inaddition, the number of pin diodes 10 a and 10 b illustrated in FIGS. 1Aand 1B can be changed to adjust the degree of soft recovery. As thenumber of pin diodes increases, the effect of soft recovery is improved.In addition, in the pin diodes 10 a and 10 b, the impurity concentrationof each n⁻ region 4 c is suppressed to a sufficiently small value toensure the breakdown voltage. Therefore, the position where the pindiodes 10 a and 10 b are arranged is not limited and the n⁻ regions 4 cmay be adjacent to each other, with the n-type drift region 4 ainterposed therebetween.

In the super junction MOSFET 50 illustrated in FIG. 1A, the second n⁺buffer layer 2 functions as a carrier reservoir during a reverserecovery operation of the super junction MOSFET 50. A carrier dischargetime is lengthened to further increase the reverse recovery time and asoft recovery waveform is obtained.

FIG. 7 is an example of a cross-sectional view illustrating a mainportion of the plane pattern of the parallel pn layer 4 which is cutalong the plane parallel to the surface of the substrate in the superjunction MOSFET 50 according to the invention. FIG. 8A is across-sectional view illustrating the main portion taken along thedashed line B-B′ of FIG. 7 and FIG. 8B is a cross-sectional viewillustrating the main portion taken along the dashed line C-C′ of FIG.7. FIG. 9 is another example of the cross-sectional view illustratingthe main portion of the plane pattern of the parallel pn layer 4 whichis cut along the plane parallel to the surface of the substrate in thesuper junction MOSFET 50 according to the invention. FIG. 10A is across-sectional view illustrating the main portion taken along thedashed line B-B′ of FIG. 9 and FIG. 10B is a cross-sectional viewillustrating the main portion taken along the dashed line C-C′ of FIG.9. In FIG. 7, the cross-sectional view taken along the dashed line A-A′corresponds to FIG. 1A, the cross-sectional view taken along the dashedline B-B′ corresponds to FIG. 8A, and the cross-sectional view takenalong the dashed line C-C′ corresponds to FIG. 8B. In FIG. 9, thecross-sectional view taken along the dashed line A-A′ corresponds toFIG. 1A, the cross-sectional view taken along the dashed line B-B′corresponds to FIG. 10A, and the cross-sectional view taken along thedashed line C-C′ corresponds to FIG. 10B.

The plane pattern of the parallel pn layer 4 illustrated in FIG. 7 has astripe shape which extends in a direction perpendicular to the directionin which the n-type drift regions 4 a and the p-type partition regions 4b are arranged in a line. In the plane pattern of the parallel pn layer4 illustrated in FIG. 9, the p-type partition regions 4 b and the n⁻regions 4 c are arranged in a lattice shape and each of the p-typepartition regions 4 b and the n⁻ regions 4 c is surrounded by the n-typedrift region 4 a. As described above, it is possible to appropriatelychange the number of n⁻ regions 4 c to be arranged. In addition, in FIG.7 and FIG. 9, the n⁻ region 4 c is not formed at the edge of theelement. A field insulating film 18 is provided on the surface of theparallel pn layer at the edge of the element. In addition, a channelstopper region 14 is provided in the outermost circumference of the edgeof the element. A channel stopper electrode 16 is provided so as to beelectrically connected to the channel stopper region 14.

Next, the characteristics of the vertical super junction MOSFET 50 witha breakdown voltage of about 600 V will be described in detail below.The dimensions and impurity concentration of each layer and each regionwill be described in brief below. The thickness of the parallel pn layer4 in the depth direction (hereinafter, the thickness means a distancefrom the substrate in the depth direction) is 36.0 μm, the pitch betweenthe parallel pn layers 4 is 12.0 μm, the width of each of the n-typedrift region 4 a and the p-type partition region 4 b is 6.0 μm, and theimpurity concentration of each region is 3.0×10¹⁵ cm⁻³. The first n⁻buffer layer 3 provided immediately below the parallel pn layer 4 (drainside) had a thickness of 9 μm and an impurity concentration of 1.0×10¹⁵cm⁻³ which was lower than that of the n-type drift region 4 a. Thesecond n⁺ buffer layer 2 provided below the first n⁻ buffer layer 3 wasset to a thickness of 15 μm and an impurity concentration of 1.0×10¹⁶cm⁻³, which was higher than that of the n-type drift region 4 a, suchthat a depletion layer was not spread even during a reverse recoveryoperation. In addition, the impurity concentration of the n⁺⁺ drainlayer 1 was 2.0×10¹⁸ cm⁻³.

FIG. 4 is a diagram illustrating different carrier lifetimedistributions of the super junction MOSFET according to Embodiment 1 ofthe invention. FIGS. 4(b) to 4(d) are diagrams illustrating theschematic carrier lifetime distributions of the vertical super junctionMOSFET 50 illustrated in FIG. 4(a). In each case, the carrier lifetimeof the second n⁺ buffer layer 2 is not controlled or is not shorter thanthat of the parallel pn layer 4 and the first buffer layer 3. Thecarrier lifetime of any one or all of the regions other than the secondbuffer layer 2 is locally shortened to increase a switching speed.Basically, an electron lifetime was 1.0×10⁻⁵ seconds and a hole lifetimewas 3.0×10⁻⁶ seconds. When the carrier lifetime was shortened, theminimum value of an electron carrier lifetime was 1.0×10⁻⁷ seconds andthe minimum value of a hole carrier lifetime was 3.0×10⁻⁸ seconds. Whena sufficient number of carriers are ensured in the second n⁺ bufferlayer 2, a soft recovery waveform is obtained during the reverserecovery operation. Therefore, high-speed switching and a soft recoverywaveform are obtained by any of the distributions illustrated in FIGS.4(b) to 4(d) in which the carrier lifetime of the second n⁺ buffer layer2 is longer than that of other regions.

In order to obtain the carrier lifetime distributions illustrated inFIGS. 4(b) and 4(c), for example, proton irradiation may be performed onthe rear surface of the substrate and a heat treatment may be performedto locally control the lifetime such that the lifetime has a peak (isthe shortest) at the depth from the front surface of the parallel pnlayer 4 in FIG. 4(b) and has a peak at the depth from the rear surfaceof the parallel pn layer 4 in FIG. 4(c). In addition, when platinum (Pt)is used as a lifetime killer and ions are implanted into the rearsurface (drain layer) of the substrate and is diffused by a heattreatment, a distribution in which the carrier lifetime has the shortestgradient on the front surface side as illustrated in FIG. 4(d) isobtained since platinum is likely to be segregated on the front surfaceside of the substrate.

In order to clarify the effect of the vertical super junction MOSFET 50(FIG. 4(a)) according to the invention having the carrier lifetimedistribution illustrated in FIG. 4(b), the recovery waveform of thesuper junction MOSFET 201 (FIG. 2A) according to the related art havingthe carrier lifetime distribution illustrated in FIG. 2B in which thecarrier lifetime was not adjusted was measured. The measurement resultis illustrated in FIG. 3. FIG. 3 is a diagram illustrating reverserecovery current waveforms corresponding to the super junction MOSFETaccording to the related art illustrated in FIG. 2 and the superjunction MOSFET according to Embodiment 1 of the invention illustratedin FIG. 1. FIG. 3 illustrates the simulation results of the reverserecovery current waveform when a power supply voltage is 400 V, aforward current is 20 A, and a change in reverse current over time is100 A/μs for the super junction MOSFETs 50 and 201. For the superjunction MOSFET 50 illustrated in FIG. 4(a), helium (He) was used as thelifetime killer, ions were implanted into the rear surface (drain layer)of the substrate, and a heat treatment was performed to control thelifetime. In addition, a concentration profile in which concentrationhad a peak at a depth of 8 μm from the source-side surface of theparallel pn layer 4 was set. The area of the pin diode 10 a in theactive region of the super junction MOSFET 50 illustrated in FIG. 4(a)was equal to that of the MOSFET region 20.

As can be seen from FIG. 3, the super junction MOSFET 201 according tothe related art has a hard recovery waveform in which a reverse recoverytime trr1 is long and a reverse recovery current has a high peak Irp1,increases rapidly, and largely oscillates. The reason is that, in aforward blocking state, carriers are likely to be depleted with thespreading of the depletion layer during reverse recovery since both thesecond buffer layer and the built-in pin diode are not provided.

In contrast, the super junction MOSFET 50 according to the invention(which is illustrated as Embodiment in FIG. 3) includes the built-in pindiode and the second buffer layer which has a higher impurityconcentration than the drift region in the parallel pn layer. Accordingto this structure, a large number of carriers are injected by the pindiode and the second buffer layer functions as a carrier reservoirduring a reverse recovery operation, which results in an increase in thetotal number of carriers. Therefore, the amount of reverse recoverycurrent (Irp) increases and the reverse recovery time is lengthened. Asa result, a soft recovery waveform is obtained.

From the above-mentioned results, in Embodiment 1, the soft recoverywaveform of the super junction MOSFET is obtained, the reverse recoveryoperation is performed at a high speed, and loss is reduced. Inaddition, in Embodiment 1 of the invention, after the second n⁺ bufferlayer 2 and the n-type first buffer layer 3 are formed on thehigh-concentration n⁺⁺ drain layer 1 (only the n-type first buffer layer3 is formed in the super junction MOSFET 51), the parallel pn layer 4 isformed by a multi-stage epitaxial method in which epitaxial growth andphotolithography are repeatedly performed a plurality of times tosequentially stack the parallel pn layers 4 in the same pattern to anecessary thickness. In addition, a trench filling method may be usedinstead of the multi-stage epitaxial method. When the parallel pn layer4 is formed by the trench filling method, first, the second n⁺ bufferlayer 2, the n-type first buffer layer 3, and a drift layer with anecessary thickness are formed on the high-concentration n⁺⁺ drain layer1 by epitaxial growth. Then, a vertical trench with a depthcorresponding to the thickness of the parallel pn layer is formed byanisotropic etching and an n⁻ silicon layer, which will be the n⁻ region4 c, is formed in the trench by epitaxial growth so as to fill up thetrench. Then, the surface is planarized such that the drift layer isexposed. Then, a vertical trench with a depth corresponding to thethickness of the parallel pn layer is formed again and a p-type siliconlayer, which will be the p-type partition region 4 b, is formed byepitaxial growth. In this way, the parallel pn layer 4 is formed. A MOSgate structure, the source electrode 12, and the rear-surface-side drainelectrode 13 are formed on the parallel pn layer 4 which is formed byany of the above-mentioned methods. In this way, a wafer process for thesuper junction MOSFET according to Embodiment 1 of the invention isalmost completed. In addition, the manufacturing method according to therelated art can be applied to the method for manufacturing the parallelpn layer 4 and the subsequent wafer process.

In general, in a power diode, as a method for shortening the carrierlifetime, a method has been used which introduces the lifetime killerfor forming a level in the band gap, using, for example, the addition ofheavy metal, such as gold (Au) or platinum (Pt) or irradiation withcharged particles, such as electron beams or protons. As such, when thelifetime killer is introduced, the extinction of carriers in the diodeis accelerated during the reverse recovery operation and the peakcurrent Irp or the reverse recovery time trr during reverse recovery isreduced. As a result, loss can be reduced during reverse recovery. Sincethe super junction MOSFET also includes the built-in diode, theabove-mentioned structure in which the lifetime killer is introduced toobtain the carrier lifetime distributions illustrated in FIGS. 4(b) to4(d) is effective in increasing the operation speed and reducing reverserecovery loss.

In the super junction MOSFET 50 according to the invention, the secondbuffer layer 2 having a higher impurity concentration than the n-typedrift region 4 a of the parallel pn layer 4 is formed below the firstbuffer layer 3. In addition, the carrier lifetime of the first bufferlayer 3 and the parallel pn layer 4 is adjusted to be shorter than thecarrier lifetime of the second buffer layer 2. When the carrier lifetimeis adjusted in this way, the recovery waveform can be gently raised anda soft recovery waveform can be obtained.

As a method for locally controlling the lifetime, the addition of heavymetal, such as gold or platinum, or irradiation with charged particles,such as protons, can be performed. Heavy metal can be added up to thefirst buffer layer 3 by the implantation of heavy metal ions into thesurface close to the source region 7 and a heat treatment. In addition,after the source electrode 12 is formed, the opposite side (rearsurface) of the substrate can be ground and the first buffer layer 3 andthe second buffer layer 2 can be formed. Then, heavy metal ions orcharged particles can be radiated to the surface of the second bufferlayer 2. In addition, the local lifetime control can be combined with acontrol process of uniformizing the lifetime, such as electron beamirradiation.

The impurity concentration and thickness of the second buffer layer 2are adjusted to make the second buffer layer 2 function as a carrierreservoir which prevents the depletion layer from reaching the n⁺⁺ drainlayer 1 even when the super junction MOSFET 50 is in a forward blockingstate. Therefore, even during the reverse recovery operation, thecarriers in the drift layer are not depleted and the reverse recoverywaveform can be gently raised.

According to the above-described Embodiment 1, a portion of thepartition region 4 b in the super junction MOSFETs 50 and 51 is replacedwith the region 4 c which has the same conductivity type as the driftregion 4 a and has a lower impurity concentration than the drift region4 a. Therefore, it is possible to achieve soft recovery. In addition, inthe super junction MOSFET 50, the buffer layer includes two layers, thatis, the first buffer layer 3 and the second buffer layer 2 and thelifetime killer is introduced such that the lifetime of the first bufferlayer 3 and the parallel pn layer 4 is shorter than that of the secondbuffer layer 2. Therefore, it is possible to further improve softrecovery, to reduce the peak current Irp or the reverse recovery timetrr during reverse recovery, and to reduce loss during reverse recovery.

EXPLANATIONS OF LETTERS OR NUMERALS

-   1 n⁺⁺ DRAIN LAYER (FIRST-CONDUCTIVITY-TYPE HIGH-CONCENTRATION    SEMICONDUCTOR SUBSTRATE)-   2 SECOND BUFFER LAYER-   3 FIRST BUFFER LAYER-   4 PARALLEL pn LAYER-   4 a n-TYPE DRIFT REGION-   4 b p-TYPE PARTITION REGION-   5 p-TYPE BASE REGION-   6 pn JUNCTION-   10 a, 10 b PIN DIODE-   50, 201 SUPER JUNCTION MOSFET-   101 IGBT-   103 COLLECTOR JUNCTION-   301 MOSFET-   401 DIODE-   1000 INVERTER CIRCUIT

What is claimed is:
 1. A method for manufacturing a semiconductor deviceincluding a high-concentration buffer layer of a first conductivity typewhich is provided on a first main surface of a drain layer of the firstconductivity type and has a higher impurity concentration than a driftregion of the first conductivity type, a low-concentration buffer layerof the first conductivity type which is provided on thehigh-concentration buffer layer and has a lower impurity concentrationthan the drift region, and a parallel pn layer which is provided on thelow-concentration buffer layer and in which the drift region of thefirst conductivity type and a partition region of a second conductivitytype are alternately arranged, at least one of the partition regionsbeing replaced with a first-conductivity-type region having a lowerimpurity concentration than the drift region, the method comprising: astep of adjusting a carrier lifetime of the parallel pn layer to beshorter than a carrier lifetime of the high-concentration buffer layer,using addition of heavy metal or irradiation with a charged particle. 2.A method for manufacturing a semiconductor device including a parallelpn layer which is provided over a first main surface of a drain layer ofa first conductivity type and in which a drift region of the firstconductivity type and a partition region of a second conductivity typeare alternately arranged, at least one of the partition regions beingreplaced with a first-conductivity-type region having a lower impurityconcentration than the drift region, the method comprising: a step offorming a high-concentration buffer layer of the first conductivitytype, which has a higher impurity concentration than the drift region,on the first main surface of the drain layer; a step of forming alow-concentration buffer layer of the first conductivity type, which hasa lower impurity concentration than the drift region, on thehigh-concentration buffer layer; a step of forming the parallel pn layeron the low-concentration buffer layer; and a step of adding heavy metalor radiating a charged particle to the parallel pn layer such that acarrier lifetime of the parallel pn layer is shorter than a carrierlifetime of the high-concentration buffer layer.
 3. A method formanufacturing a semiconductor device including a parallel pn layer whichis provided over a first main surface of a drain layer of a firstconductivity type and in which a drift region of the first conductivitytype and a partition region of a second conductivity type arealternately arranged, at least one of the partition regions beingreplaced with a first-conductivity-type region having a lower impurityconcentration than the drift region, the method comprising: a step offorming the parallel pn layer on a front surface side of a semiconductorsubstrate; a step of forming an element structure on the parallel pnlayer on the front surface side of the semiconductor substrate; a stepof forming a low-concentration buffer layer of the first conductivitytype, which has a lower impurity concentration than the drift region, ona rear surface side of the semiconductor substrate; a step of forming ahigh-concentration buffer layer of the first conductivity type, whichhas a higher impurity concentration than the drift region, at a positionthat is shallower than the low-concentration buffer layer from a rearsurface of the semiconductor substrate; and a step of adding heavy metalor radiating a charged particle to the high-concentration buffer layersuch that a carrier lifetime of the parallel pn layer is shorter than acarrier lifetime of the high-concentration buffer layer.